X-ray excited fluorescence and phosphorescence of CaSO4: Sm phosphors

Calcium sulphate phosphors activated with samarium as an impurity with various percentage composition have been prepared and their fluorescence rise and phosphorescence decay behaviours have been studied. The growth curve is investigated to study the density of traps. The process of filling of traps during x-ray excitation is revealed. The decay curves are analysed to study the distribution of traps, the nature of decay and kinetics involved in the luminescence process. The concentration dependence of fluorescence emission of the phosphors is also discussed.     

Decay of quantum correlations in atom optics billiards with chaotic and mixed dynamics

We perform echo spectroscopy on ultracold atoms in atom-optics billiards to study their quantum dynamics. The detuning of the trapping laser is used to change the "perturbation", which causes a decay in the echo coherence. Two different regimes are observed: first, a perturbative regime in which the decay of echo coherence is nonmonotonic and partial revivals of coherence are observed in contrast with the predictions of random matrix theory. These revivals are more pronounced in traps with mixed dynamics as compared to traps where the dynamics is fully chaotic. Next, for stronger perturbations, the decay becomes monotonic and independent of the strength of the perturbation. In this regime no clear distinction can be made between chaotic traps and traps with mixed dynamics.     

Temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO3

The temperature dependence of two-beam coupling and dark decay in photorefractive BaTiO₃ is reported. We show that the competition between deep and shallow traps depends on temperature and writing intensity, and influences two-beam coupling and dark decay. The dynamics of dark decay, characterized by a fast decay of partial erasure and a subsequent slow decay, is influenced by the presence of deep and shallow traps. Partial erasure, due to thermal excitation of charges from the shallow traps, decreases with temperature and increases with writing intensity. The time constant of the slow decay, due to thermal excitation of charges from the deep traps, depends strongly on temperature, but not on the writing intensity. At room temperature, the existence of deep and shallow trap leads to intensity-dependent photorefractive gains. As temperature increases, the influence from the shallow trap decreases, and the photorefractive gain becomes independent of the intensity. However, at much higher temperatures (100°C), the photorefractive gain resumes its dependence on intensity due to an increase in dark conductivity at elevated temperature.     

Temperatur- und Zeitabhängigkeit der Verzögerten Elektronenemission

Occupied traps responsible for delayed electron emission show an exponential decay. From the form of a single glowmaximum one can determine the temperature dependence of the decay constant of the traps of one sort. The decay constant is shown to have the form

$$\lambda (T) = \lambda _0 \exp ( - \varepsilon /kT).$$     

纳米硫化镍增感的溴化银微晶中光电子衰减特性研究

利用微弱信号的微波吸收相敏检测技术, 获得了纳米硫化镍增感的立方体溴化银乳剂中, 在增感时间增加的条件下, 自由光电子和浅束缚光电子的时间衰减曲线.分析了采用纳米硫化镍进行增感的溴化银乳剂中光生电子随增感时间衰减的过程, 讨论了卤化银晶体中电子陷阱对光电子运动行为的影响, 分析了电子陷阱效应及陷阱深度同增感时间之间的关系. 通过未增感样品与增感样品的衰减曲线对比, 得到了在此实验条件下的最佳增感时间为80min. 关键词： 纳米硫化镍 衰减时间 电子陷阱 寿命     

Haftstellenerzeugung und Leuchtmechanismus bei den Wolframat-Phosphoren

Thermoluminescence investigations of several tungstate and molybdate phosphors, excited by UV-, cathode- und X-rays, show that cathode- and X-rays are able to produce electron traps in the centres. Thus, after irradiation an amplification of the UV-light sum can be observed, and the form of the glow curves differs considerably after the various excitations. Theoretical conceptions about the trap production are confirmed quantitatively by measuring the variation of the light sum with irradiation time at various kinds of excitation. However, the produced traps influence the simple exponential decay after UV-excitation and lead to the appearance of a second exponential relation in the decay curve. These decay measurements can also be verified by theoretical considerations about the kinetics of possible transitions. It is supposed that the traps, which can be healed by annealing, are situated in the electron configuration of the tungstate complex; a model for the luminescence centre is discussed.     

Simultaneous determination of trap depth and the ratio of the rate of recombination to that of capture from thermo-luminescence

Thermo-luminescence (TL) is a kind of luminescence decay measured with varying temperature. In the process of TL the decay parameter itself involves the temperature effect of traps. Thus the trap depth is inseparable from the decay parameter. There are two separate peaks in the TL curve of ZnS:Cu,Co if the measurement starts from liquid nitrogen temperature. In the experiment we started from zero Celsius temperature to isolate the deeper traps. We have proposed and realized three methods for simultaneous determination of trap depth and decay parameter based on the quasi-equilibrium model and experimental data. If we treat the case of kinetic order \alpha =1 as \alpha =2, the error might be as large as 100%.     

ZnS中电子陷阱能级对光生电子弛豫过程的影响

采用微波吸收法,测量了在不同助熔剂条件及不同气氛下烧制的ZnS材料受到超短激光脉冲激发后的光电子衰减过程,并且测量了材料的热释光曲线。样品A采用过量的SrCl作为助熔剂,在1150℃下灼烧制备而成;其热释光曲线显示材料中有浅电子陷阱,电子陷阱密度小,光生电子衰减过程为双指数衰减过程,快过程寿命为45ns,慢过程寿命为312ns。样品B中加入了少量的NaCl作为助熔剂;热释光曲线显示有浅电子陷阱和深电子陷阱,且都有较高的密度,其光电子寿命为1615ns。在NH₄Br气氛中烧制样品C,热释光谱显示只有浅电子陷阱形成,光电子寿命为1413ns。结果表明材料的光电子寿命和浅电子陷阱密切相关,浅电子陷阱密度越大,光生电子寿命越长,深电子陷阱对光生电子瞬态过程影响很小。     

低密度聚乙烯热压成型过程中的空间电荷

借助开路热刺激放电(TSD)电流及原位实时电荷TSD和电荷等温衰减测量，研究了低密度聚乙烯(LDPE)在热压成型过程中所产生的空间电荷特性.结果表明具有良好室温稳定性的成型电荷被束缚在两类陷阱能级中：浅阱和深阱，其陷阱中心深度分别约为0.92eV和1.31eV.初步的分析进一步表明了它们应该分别位于试样的表层和体内，为近表面陷阱和体陷阱. 关键词： 低密度聚乙烯(LDPE) 热压成型 空间电荷 热刺激放电(TSD)     

卤化银微晶中光电子衰减谱的特性

光电子在卤化银材料潜影形成过程中发挥着重要的作用 ,光电子衰减行为在很大程度上决定于卤化银的晶体结构。采用光学与微波双共振技术 ,测量了自由光电子和浅俘获光电子的衰减谱 ,得到了卤化银中电子陷阱的密度和深度分布。以自由光电子寿命为纽带 ,通过分析掺杂卤化银晶体中电子陷阱的分布情况 ,可以确定浅电子陷阱掺杂剂的最佳掺杂浓度。     

Random walks with imperfect trapping in the decoupled-ring approximation

We investigate random walks on a lattice with imperfect traps. In one dimension, we perturbatively compute the survival probability by reducing the problem to a particle diffusing on a closed ring containing just one single trap. Numerical simulations reveal this solution, which is exact in the limit of perfect traps, to be remarkably robust with respect to a significant lowering of the trapping probability. We demonstrate that for randomly distributed traps, the long-time asymptotics of our result recovers the known stretched exponential decay. We also study an anisotropic three-dimensional version of our model. We discuss possible applications of some of our findings to the decay of excitons in semiconducting organic polymer materials, and emphasize the crucial influence of the spatial trap distribution on the kinetics. Received 23 July 2001 / Received in final form 14 May 2002 Published online 13 August 2002     

Random walks in a random field of decaying traps

We study random walks on ^d (d 1) containing traps subject to decay. The initial trap distribution is random. In the course of time, traps decay independently according to a given lifetime distribution. We derive a necessary and sufficient condition under which the walk eventually gets trapped with probability 1. We prove bounds and asymptotic estimates for the survival probability as a function of time and for the average trapping time. These are compared with some well-known results for nondecaying traps.     

Diffusion and survival in a medium with imperfect traps

This paper deals with independent particles diffusing on a line with traps at random positions. It is shown how the long-time decay of the survival probability is exhanced when particles do not necessarily disappear upon hitting a trap. The results are compared with predictions for a model where particles are either absorbed or reflected by traps.     

The Nature of Trapping Centers in Amorphous Biphenyl-Containing Polymer

Study of optical properties of methyl methacrylate and 4-vinylbiphenyl copolymers in a rigid amorphous polymer allowed establishing the non-excimer nature of energy traps in a polymer chain. These traps are probably related to possibility of collective states formed by macromolecule’s chromophore groups. The shortening of the trap states decay time in 4-vinylbiphenyl homopolymer comparing to copolymer is observed.     

感绿光谱增感染料在氯化银微晶表面的电子陷阱效应

光谱增感技术可使卤化银感光材料实现对全波段感光，同时光谱增感技术在现代光信息记录与存储、光电器件、太阳能转换与存储等领域具有重要的应用.应用微波吸收介电谱技术研究了立方体氯化银吸附感绿菁染料后的光电子衰减特性，建立了氯化银光电子衰减动力学模型，根据此模型结合光电子衰减实验结果对光谱增感染料吸附在卤化银表面的电子陷阱效应进行了分析.研究结果表明：当染料以单分子态吸附在卤化银表面时，染料起浅电子陷阱效应；染料以J聚集体吸附在卤化银表面时，染料起到了深电子陷阱效应.浅电子陷阱与深电子陷阱效应的临界浓度为每40g 关键词： 感绿染料 氯化银 光电子衰减 电子陷阱效应     

Survival probabilities in coherent exciton transfer with trapping

In the quest for signatures of coherent transport we consider exciton trapping in the continuous-time quantum walk framework. The survival probability displays different decay domains, related to distinct regions of the spectrum of the Hamiltonian. For linear systems and at intermediate times the decay obeys a power law, in contrast with the corresponding exponential decay found in incoherent continuous-time random walk situations. To differentiate between the coherent and incoherent mechanisms, we present an experimental protocol based on a frozen Rydberg gas structured by optical dipole traps.     

TL, OSL, and phototransferred TL in beta-irradiated anion-defective Al2O3

The behavior of TL, OSL and PTTL under thermal and/or optical stimulation in beta-irradiated -Al₂O₃ was investigated. The noticeable change of the shape of the main TL peak after thermal or optical stimulation clearly shows that this peak is related to at least two traps. The OSL curves recorded after preheating up to a given increased end temperatures are described by two exponential decays curves. The first one is associated to the decay of the traps responsible for the main TL peak. The second exponential is related with the phototransfer from the deep to the main traps. A simple model of the OSL process, based on simultaneous refilling of shallow traps and their reverse filling due to phototransfer from deep traps, under illumination with light is proposed. The model describes well the obtained experimental data.     

Influence of pre-measurement thermal treatment on OSL of synthetic quartz measured at room temperature

Much effort has been made to study the influence of pre-measurement thermal treatment and ionizing radiation on quartz specimens owing to its use in a large number of applications. Optically stimulated luminescence (OSL) being a structured and sensitive phenomenon promises to correlate the responsible color center and luminescence emission. OSL studies on quartz with such conditions can reveal many significant results.The aim of the present investigation is to understand the effect of annealing temperature on OSL characteristics of synthetic quartz recorded at room temperature. At identical annealing duration and β-dose, the shape of OSL decay curve remains non-exponential; when specimens annealed at lower temperature (∼400 °C). The shape of decay curve changes to exponential in nature along with rise in OSL intensity when the specimen was given higher temperature of annealing (>400 °C). The effects of such protocol on pattern of OSL sensitivity as well as area under the OSL decay curve are also presented here. The presence of shallow traps, when OSL decay curve was recorded at room temperature seems to be responsible for the changes in OSL pattern. The influence of shallow traps is attributed to non-exponential decay of OSL recorded at room temperature.     

On the temperature dependence of the phosphorescence of CaF2 : Mn phosphor in different atmospheres

The temperature dependence inthe region of 24–150°C of the phosphorescence of CaF₂ : Mn phosphor in the atmospheres of nitrogen gas, vacuum and moisture-saturated air were studied. A decay formula was found to adequately describe the rate of decay of the trapped electrons — which is assumed to be proportional to the amount of phosphorescence observed — in thermal traps. In nitrogen gas and vacuum the decay constants of the trapped electrons are dependent only on the storage temperature and not on the storage atmosphere of the phosphor. A slight effect of water on the rate of decay of the trapped electrons was also observed.